Suppression of corrosion of iron in sodium

ABSTRACT

A METHOD OF REDUCING THE CORROSIVE EFFECTS OF MOLTEN SODIUM ON THE WALLS OF AN IRON BEARING CONTAINMENT STRUCTURE COMPRISING THE ADDITION OF NICKEL OR CHROMIUM TO THE SODIUM IN SOLUTION NEAR TO OR AT THE POINT OF THE HIGHEST TEMPERATURE IN THE SODIUM SYSTEM.   D R A W I N G

Apr]! 10, 5 |5AACS ET AL SUPPRESSION OF CORROSION OF IRON IN SODIUM Filed Deg.

WITHOUT Ni WITH Ni T= 333 hrs DiSTANCE, INCHES 3,726,642 SUPPRESSION OF CORROSION OF IRON IN SODIUM Hugh S. Isaacs, Shoreham, and Anthony J. Romano,

Kings Park, N.Y., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 29, 1971, Ser. No. 213,644 Int. Cl. C23f 11/08, 14/02 US. CI. 21-25 R Claims ABSTRACT OF THE DISCLOSURE A method of reducing the corrosive effects of molten sodium on the walls of an iron bearing containment structure comprising the addition of nickel or chromium to the sodium in solution near to or at the point of the highest temperature in the sodium system.

SOURCE OF THE INVENTION The invention described herein was made in the course of, or under a contract with the US. Atomic Energy Commission.

BACKGROUND OF THE INVENTION In sodium cooled nuclear reactors, corrosion of the materials in contact with the liquid metal is a severe problem to which extensive attention has been paid. In the case of iron-bearing walls such as that in stainless steel pipe, the sodium has a tendency due to the kinetics of the system to remove the nickel and chromium from the wall followed after depletion thereof by removal of the iron component. It is the removal of the iron component which is of most concern in sodium systems. The tendency of the elements to reach chemical equilibrium also plays a part in this process.

It is known that the presence of oxygen in small amounts in the liquid metal is an important factor influencing and promoting corrosion. Consequently, efforts have been made to monitor and control the oxygen content of the liquid sodium for the purpose of minimizing corroding conditions. While this does have the effect of reducing the extent of the corrosion problem, the oxygen can not be eliminated entirely, and in any event the problem can be reduced only partially by the reduction or elimination of the oxygen.

SUMMARY OF THE INVENTION The present invention tends to suppress the removal of iron, that is, the corrosion of iron bearing materials in a liquid sodium environment to an extent which has not been attained heretofore. In accordance with the principles of this invention, the liquid sodium is enriched with nickel or chromium whose presence has been found to inhibit removal by the sodium of the iron component of the metal wall.

It is thus a principal object of this invention to provide for the reduction of corrosion in the ferrous alloy containment of liquid sodium.

Other objects and advantages of this invention will hereinafter become obvious from the following description of preferred embodiments of this invention.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrates graphically the reduced level of atent 3,726,642 Patented Apr. 10, 1973 DESCRIPTION OF THE PREFERRED EMBODIMENTS In a system in which liquid sodium is flowing through a ferrous-alloy containment, corrosion of the iron-bearing material is suppressed by contacting the sodium with chromium or nickel near or at the highest temperature region, where corrosion is most severe. Some of the chromium or nickel is dissolved in the sodium, and after a period of time, the concentration of the additive in sodium is such that the metal will inhibit the corrosive effects mentioned previously. Experiments indicate that a minimum of 10 p.p.b. (parts per billion) of the additive in solution is required in order to obtain the desired results. There is also a tendency for the nickel or chromium to plate out or to adsorb on the walls of the containment downstream of where the chromium or nickel is placed, in cooler regions of the sodium system.

In accordance with a preferred embodiment of the invention, at or near the highest temperature region in the sodium loop a portion of the metal wall is replaced by a nickel or chromium pipe section. The metal additive is dissolved by the flowing sodium and inhibits corrosion in the region immediately downstream. The section should have sufiicient surface area exposed to the sodium to provide 10 p.p.b. of the additive in the highest temperature region after steady state conditions are reached. In the lower temperature portion of the system (or loop) the excess nickel or chromium present in the sodium is deposited safely out on the walls of the sodium containment. Thus, by adding the nickel or chromium at or near the highest temperature in the system, the whole system is protected against the described corrosive effects of the sodium.

In an example of this invention, molten sodium was pumped through a closed loop of stainless steel pipe. In a 36" stretch of pipe where heat was added to the loop, the sodium was at 710 C. at the beginning of the section and 760 C. at the downstream end of the section. Within this test section was inserted an inner test pipe surrounded by the outer pipe, forming a 0.040" annulus. The test pipe consisted of half inch removable sections of stainless steel pipe. Over a period of 333 hours of sodium flow after a period of conditioning during which the nickel and chromium are leached out of the surface of the walls, corrosion of the test pipe was measured by removing the half inch sections and noting the weight loss, shown by curve A in the figure. The test run was then repeated with fresh sodium, test sections, and identical conditions except that the stainless steel ring located 12." from the upstream end of the test section was replaced by a nickel /2" section. After 333 hours of sodium flow, the sections were removed to measure weight loss and curve B indicates the extent of the reduced level of wall removal which had taken place. Studies of chromium in place of nickel indicate that chromium has the same inhibiting effect on the corrosiveness of the sodium.

Instead of use of a nickel or chromium section, it is understood that the additive may be inserted in other appropriate ways such as by adding powdered metal to the sodium.

What is claimed is:

1. The method of suppressing corrosion of iron-bearing material in contact with flowing liquid sodium comprising the step of adding in solution to said sodium a metal selected from the group consisting of chromium and References Cited nickel. N TED TAT PAT NT 2. The method according to claim 1 in which the metal 2 943 034 i z E S 252 71 X I d f r a 12: 3:5 or near the hlghest temperature reglon 5 035 011 5/1962 Mill r 252-71 3. The mehod according to claim 2 in which the metal 3343380 3/1966 Bohlmann et 176*38 X is added to at least 10 p.p.b.

4. The method of claim 3 in which the iron-bearing MORRIS WOLK Pnmary Exammer material in contact with the flowing sodium forms a closed D. G. MILLMAN, Assistant Examiner system, and the metal to be added in solution is inserted 10 in solid form into the flowing sodium. us

5. The method of claim 4 in which the metal is added 25271 in powdered form. 

